Hitherto, recording materials utilizing a color reaction of a color former with a compound which forms a color upon contacting said color former (hereinafter, referred to as a developer) have been well known. Examples thereof include pressure-sensitive copying paper, heat-sensitive recording paper, and electric current sensitive recording paper, etc., which have been described in U.S. Pat. Nos. 2,712,507, 2,730,456, 2,730,457, 3,418,250, 3,432,327, 3,981,821, 3,993,831, 3,996,156, 3,996,405 and 4,000,087, etc.
In these recording materials, clay minerals such as acid clay, bentonite or kaolin, etc., phenolformaldehyde novolak, metal salt of novolak and zinc di-tert-butylsalicylate, etc., have been used as the developer.
Further, nearly colorless compounds having a partial skeleton comprising a lactone, lactam, sultone, spiropyran, ester or amide, etc., which undergo ring-cleavage or dissociation upon contact with the developer have been used as the color former. Examples thereof include Crystal Violet lactone, benzoyl Leucomethylene Blue, Malachite Green lactone, Rhodamine B lactam, 1,3,3-trimethyl-6'-ethyl-8'-butoxyindolinobenzospiropyran, and 4,4'-bisdimethylaminodiphenylmethyl-p-dodecylphenyl sulfone, etc.
These color formers are sometimes used as a combination of two or more thereof.
In the case of using developers as described above, the developing ability of the developer sheet deteriorates over a period of time when the developer sheet is allowed to remain exposed to the atmosphere. Since deterioration of the developing ability has a remarkable influence upon trade value, improvement of the developing ability stability has been desired.
Further, images on the developer layer may disappear due to influences such as organic solvent vapor, a plasticizer, etc. This drawback is one of unsuitable phenomena which are desired to be overcome.
Furthermore, when the developer layer is wetted with water while contacting a color former layer, images on the developer layer cannot be observed. This drawback is another matter with respect to which improvement is desired.
It has already been found by the present inventors that recording materials having excellent solvent resistance and excellent water resistance and which undergo less deterioration of developing ability over a period of time can be prepared by using a particular type of phenolic compound such as a bisphenol compound or a phenolic resin together with acid-treated activated clay (e.g., British Pat. No. 1,356,402).
However, though the recording materials using the bisphenol compound or phenolic resin together with the acid-treated activated clay have the above-described advantages, they have drawback in that the developer layer tends to yellow over a period of time or by exposure to sunlight or other sources of ultraviolet rays. Therefore, it is required to overcome such a drawback.